Polyimide precursor and method and composition for preparing it

Abstract

A composition is disclosed of a dianhydride and a tetracarboxylic acid in the proportions of about 50 to about 70 mole % dianhydride to about 30 to about 50 mole % tetracarboxylic acid, a stoichiometric amount of an aromatic diisocyanate, and a solvent for the dianhydride, the tetracarboxylic acid, and the diisocyanate. A polyimide precursor is prepared by heating the composition at about 30° to 70° C until the evolution of carbon dioxide substantially ceases, and then at about 30° to about 100° C until its viscosity is about Q to about Z6 on the Gardner-Holdt viscosity scale at 18% solids. The polyimide precursor can be spread on a substrate and cured to produce a polyimide film or coated on a wire and cured to produce a polyimide wire enamel.

We claim: 
     
       1. A method of making a soluble polyimide precursor comprising: 1. preparing a composition which comprises: (a) a dianhydride and a tetracarboxylic acid in the proportions of about 50 to about 70 mole percent dianhydride and about 30 to about 50 mole percent tetracarboxylic acid;   (b) a stoichiometric amount of an aromatic diisocyanate; and   (c) a solvent for said dianhydride, said tetracarboxylic acid, said aromatic diisocyanate, and said polyimide precursor in an amount sufficient to make a solution having a solids content of about 12 to about 25 percent;     2. heating said composition at about 30 to about 70° C until the evolution of carbon dioxide substantially ceases; and   3. heating said composition at about 30 to about 100° C until its viscosity is about .[.Z.]. .Iadd.Q .Iaddend.to about Z6 on the Gardner-Holdt viscosity scale at 18 percent solids.   
     
     
       2. A method according to claim 1 wherein the proportion of dianhydride to tetracarboxylic acid is about 60 to about 70 mole % dianhydride and about 30 to about 40 mole % tetracarboxylic acid. 
     
     
       3. A method according to claim 1 wherein said composition in step (2) is heated at about 40° to about 70° C until the evolution of carbon dioxide substantially ceases. 
     
     
       4. A method according to claim 1 wherein said composition in step (3) is heated at about 70 to about 90° C until its viscosity is about Z to about Z6 on the Gardner-Holdt viscosity scale at 18 percent solids. 
     
     
       5. A method according to claim 1 wherein said dianhydride and said tetracarboxylic acid are aromatic. 
     
     
       6. A method according to claim 5 wherein said dianhydride is selected from the group consisting of pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and mixtures thereof, and said tetracarboxylic acid is selected from the group consisting of pyromellitic tetracarboxylic acid, 3,3',4,4'-benzophenone tetracarboxylic acid, and mixtures thereof. 
     
     
       7. A method according to claim 1 wherein said aromatic diisocyanate is p,p'-diisocyanatodiphenylether. 
     
     
       8. A method according to claim 1 wherein said aromatic diisocyanate is p,p'-diisocyanatodiphenylmethane. 
     
     
       9. A method according to claim 1 wherein after step (3) said composition is spread on a substrate and cured to form a film. 
     
     
       10. A method according to claim 1 wherein after step (3) said composition is coated on a paper or fabric and cured to form an insulating sheet. 
     
     
       11. A method according to claim 1 wherein after step (3) said composition is coated on a wire and cured to form a wire enamel. 
     
     
       12. A method according to claim 11 wherein said composition is cured at about 275° to about 300° C. 
     
     
       13. A method according to claim 1 wherein said composition is formed by mixing said dianhydride with said solvent, adding water to form said tetracarboxylic acid, then adding said aromatic diisocyanate. 
     
     
       14. A composition comprising: 1. a dianhydride and a tetracarboxylic acid in the proportions of about 50 to about 70 mole % dianhydride and about 30 to about 50 mole % tetracarboxylic acid;   2. a stoichiometric amount of an aromatic diisocyanate; and   3. a solvent for said dianhydride, said tetracarboxylic acid and said diisocyanate.   
     
     
       15. A composition according to claim 14 wherein the proportion of dianhydride to tetracarboxylic acid is about 60 to about 70 mole % dianhydride and about 30 to about 40 mole % tetracarboxylic acid. 
     
     
       16. A composition according to claim 14 wherein said dianhydride and said tetracarboxylic acid are aromatic. 
     
     
       17. A composition according to claim 16 wherein said dianhydride is selected from the group consisting of pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and mixtures thereof, and said tetracarboxylic acid is selected from the group consisting of pyromellitic tetracarboxylic acid, 3,3',4,4'-benzophenone tetracarboxylic acid, and mixtures thereof. 
     
     
       18. A composition according to claim 14 wherein said aromatic diisocyanate is p,p'-diisocyanatodiphenylether. 
     
     
       19. A composition according to claim 14 wherein said aromatic diisocyanate is p,p'-diisocyanatodiphenylmethane. 
     
     
       20. An polyimide precursor made according to the method of claim 1. 
     
     
       21. A polyimide precursor according to claim 20 wherein said dianhydride and said tetracarboxylic acid are aromatic. 
     
     
       22. A polyimide precursor according to claim 21 wherein said dianhydride is selected from the group consisting of pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, and mixtures thereof, and said tetracarboxylic acid is selected from the group consisting of pyromellitic tetracarboxylic acid, 3,3',4,4'-benzophenone tetracarboxylic acid, and mixtures thereof. 
     
     
       23. A polyimide precursor according to claim 20 wherein said aromatic diisocyanate is p,p'-diisocyanatodiphenylether. 
     
     
       24. A polyimide precursor according to claim 20 wherein said aromatic diisocyanate is p,p'-diisocyanatodiphenylmethane. 
     
     
       25. A method according to claim 1 wherein after step (3) said composition is cured about 100° to about 375° C. for about 1 to about 2 hours. 
     
     
       26. A method according to claim 22 wherein said composition is cured at about 275° to about 300° C for about 1/4 to about 1/2 hour. 
     
     
       27. A method according to claim 1 wherein said solvent is N-methyl pyrrolidone. .Iadd. 28. A method according to claim 1 wherein said composition includes up to about 1% of a catalyst. .Iaddend..Iadd. 29. A composition according to claim 14 which includes up to about 1% of a catalyst. .Iaddend.